Voyager 1 has left the solar system — almost

Wire
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Mar 21, 2013 at 3:07 PM

It was welcome news to Earthlings: The Voyager 1 spacecraft had seemingly crossed a momentous threshold and become the first man-made object to enter interstellar space.

“Voyager 1 has left the solar system,” the American Geophysical Union declared Wednesday in a news release. An accompanying study published online in the organization’s journal, Geophysical Research Letters, also contained an unusually sentimental end note declaring that “we did it. Bon Voyage!”

Alas, the elation that spread through news and social media was short-lived. Voyager 1 was still in the neighborhood, NASA said, even after traveling for more than 35 years. Then the American Geophysical Union press office issued a correction of its headline, omitting any reference to the spacecraft having departed “the solar system.”

“The Voyager team is aware of reports today that NASA’s Voyager 1 has left the solar system,” said Edward Stone, Voyager project scientist at the California Institute of Technology and former chief of the Jet Propulsion Laboratory in La Canada Flintridge, where Voyager was built. “It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space.”

Though there is little doubt that the lonely probe will one day exit the solar system, scientists are discovering that the border is not as clearly defined as they expected it to be.

In the paper released Wednesday, lead author Bill Webber suggested that the probe had exited the heliosphere — that region of space dominated by solar winds and long considered to be the edge of the solar system — on Aug. 25, 2012.

It was on that day that Voyager’s sensors registered drastic changes in radiation levels. There was a sharp drop in so-called anomalous cosmic rays — high-energy particles trapped within the “bubble” of the outer heliosphere — and a sudden surge in galactic cosmic rays from outside the solar system.

Together, those events seemed to indicate that Voyager had “crossed a well-defined boundary” and possibly entered interstellar space 11.3 billion miles from the sun, according to the paper.

“It appears that V1 has exited the main solar modulation region, revealing hydrogen and helium spectra characteristic of those to be expected in the local interstellar medium,” wrote Webber, a professor emeritus of astronomy at New Mexico State University who is involved with cosmic ray experiments on Voyager.

But scientists at NASA and elsewhere said Webber’s report did not address one of the most unexpected elements of the boundary between the solar system and interstellar space — a mysterious region that Stone and others have labeled a “magnetic highway.”

In December, the Voyager science team reported that the spacecraft had reached a place where particles from the solar wind dropped off dramatically and cosmic rays from interstellar space increased. But they did not detect an anticipated change in the direction of the magnetic field emanating from the sun.

“If we had looked at particle data alone, we would have said, ‘We’re out! Goodbye, solar system!’ ” Stamatios Krimigis, a solar physicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md., said at the time.

Instead, the scientists surmised that Voyager had reached the magnetic highway, where interstellar particles can ride in and solar system particles can ride out. Only when the craft senses that the magnetic field has changed direction will they declare that it has reached interstellar space.

“And that change of direction has not yet been observed,” Stone said Wednesday.

So the new report from Webber left his fellow Voyager scientists scratching their heads.

“We did not leave the solar system,” said Merav Opher, an astrophysicist at Boston University and member of the Voyager team. “We are simply in a new region that is completely different than what we thought.”

Webber did not return calls or email seeking comment Wednesday.

A good deal of the confusion can be traced to the American Geophysical Union’s news release. A press representative said it was a challenge to convey the significance of the study. He said he realized soon after the release was issued that it may not have been fully accurate.

“We were trying to create a headline that was meaningful to reporters and to general audiences, and I guess we overstated the conclusions a little bit,” said Peter Weiss, a public information manager for the American Geophysical Union in Washington.

The Voyager study was unusual for other reasons as well.

It was published by just two scientists and not the larger Voyager team. And one of those scientists, astrophysicist Frank McDonald of the University of Maryland, had died in August — just days after he and Webber determined that Voyager had crossed a boundary of great significance.

McDonald, who helped design instruments for many NASA space probes, collapsed from a brain aneurysm while speaking at a symposium, said Gary Zank, a space physicist who witnessed the event. He died the next day.

Webber went on to write up the pair’s findings. The end of the paper published in Geophysical Research Letters contained a rare sentimental note to his former colleague:

“Frank, we have been working together for over 55 years to reach the goal of actually observing the interstellar spectra of cosmic rays, possibly now achieved almost on the day of your passing,” he wrote. “You wanted so badly to be able to finish this article that you had already started. Together we did it. Bon Voyage!”

McDonald’s widow, Irene “Rene” McDonald, said her late husband was indeed looking forward to the day when Voyager exited the solar system.“I know that when they do confirm that they actually have gone beyond the solar system with Voyager, he’d be pleased as punch,” she said. “I know also as a scientist, he’d be extremely cautious about saying that it truly has happened.”